This application claims priority to German Application No. 100 40 828.1, filed Aug. 21, 2000, the disclosure of which is hereby incorporated herein by reference.
The invention relates to a seamed felt such as is variously employed in paper machines as a press felt to remove water from a web of paper.
For this purpose, in the paper machine the web of paper is pressed between two felts or between a felt and a roller, so that the water is removed.
For reasons of operating safety and to shorten the time during which the machine must be stopped for installation of the felts, seamed felts are increasingly being used on the paper machine in the lower and intermediate speed range and for papers with relatively low quality requirements. As a rule, these felts are composed of a woven backing fabric of coarse monofils in the longitudinal and transverse directions, with a monofil diameter in the range 0.35 mm to 0.5 mm. Onto this fabric fibres are needled in the conventional manner to form a felt-like structure.
The disadvantage of this backing-fabric concept lies in the poor anchoring of the fibres and the increased frictional wear and tear of the press felt, the tendency of the coarse backing fabric to leave marks on sensitive papers, and the low damping capacity of the felt on vibration-sensitive press positions.
In order to eliminate the problem of poor fibre anchoring, in the past attempts have been made to use curled yarns, as is described for example in the patent EP 0 502 638 A1. However, the curling of such yarns makes it difficult to work with them. Furthermore, it is difficult to produce and maintain a specific and reproducible curling of the yarns, in particular when different kinds of fibre materials are used.
A similar attempt to eliminate the above-mentioned disadvantages is disclosed in DE 39 30 315, which describes felts with braided yarns in the long direction with respect to the direction of movement of the endless band in the paper machine. Here, however, it has proved disadvantageous that on one hand the manufacture of braided yarns is elaborate and expensive, whereas on the other hand the felts made with these braided yarns show a declining elasticity and/or an impermanent or temporally unspecified stability.
The patent U.S. Pat. No. 5,514,438 describes felts for use in a paper machine, in which wound yarns are employed in the long direction with respect to the direction of movement of the endless band in the paper machine. These wound yarns consist of monofils surrounded by a layer or several layers of multifils. This embodiment, too, has so far proved to be suboptimal, because the construction of the wound yarns is very complex and hence they are complicated and expensive to manufacture.
The objective of the invention is thus to make available felts in which the fibre anchoring is improved in comparison to the known state of the art and which have a lower tendency to leave marks as well as a higher damping potential in comparison to the known state of the art.
For this purpose the invention includes the essential idea of improving the seamed felts previously used in paper machines by using structured fibres not only as the longitudinal threads of a basic textile area used as backing fabric, i.e. those aligned with the direction of movement, but also as the transverse or weft threads, which run substantially in the perpendicular direction. It further includes the idea of providing a twisted structure in which monofils, each of which in itself has a helical construction, are entwined with one another.
It has proved advantageous for the twisted structure to have a substantially round cross section. Surprisingly, it has been found particularly advantageous for this cross-sectional shape to be formed by entwining three monofils with one another, because when three monofils are used, an approximately homogeneous and substantially circular cross section is achieved over the entire length of the twisted structure. Another substantial advantage of the use of three monofils to produce the twisted structure lies in the fact that it is easy to handle threads that are not too thin, whereas the overall diameter of the twisted structure must not become too large, and this is enabled by the use of several monofils. Furthermore, three monofils provide adequate stability, so that an optimal combination of stability and flexibility is achieved.
In contrast, a twisted structure made of only two monofils has a cross section in the shape of two circles side by side, while a twisted structure made of four monofils has a substantially four-cornered shape with rounded corners. Furthermore, the diameter of the twisted structure as a whole increases, the more monofils are incorporated therein, so that the twisted structure in itself becomes more rigid and hence more difficult to work with. In principle, however, twisted threads made of five or more individual monofils are possible, in which case the diameter of each individual strand is made smaller.
The textile backing elements are constructed in at least two-ply form. This minimally two-ply backing element (see FIG. 1=duplex design) forms the basis for combinations with one or more woven fabrics which, laid over or under the backing fabric, can be connected thereto by means of needles. For special applications it is also possible to place two seamed backing fabrics (see FIG. 2=laminate) one over the other and join them together by needling or adhesive technology to form a backing element. Such backing elements make it possible to dispose between the woven layers other layers of fibres suitable for forming a felt-like structure.
According to another advantageous design, it is likewise possible to provide between the layers of the textile backing element special damping layers that have a suitable structure and are made of a material suited to the particular application.
Preferably when the textile backing element is constructed in several-ply form, at least one upper ply of longitudinal threads is connected to a lower ply, in which case the seam loop can be formed between upper and middle, upper and lower or middle and lower ply. The advantage of this and similar constructions lies in the greater thickness, lower tendency to leave marks and better damping in comparison to a two-ply or a laminated backing element. Owing to the inclusion of an additional ply of longitudinal threads by weaving technology, the textile backing element gains stability.
This stabilizing effect on the felt, combined with preservation of its mobility, in particular in the region of rollers over which the felt passes during operation of the paper machine, is reinforced by the twisted structure of the transverse threads. The twisting of the monofils makes it possible for the threads used to produce the felt to penetrate into and/or through the twisted structure between the monofils, and thus to be optimally anchored in the backing fabric. When plain monofils are used instead of a twisted structure, such anchoring is impossible.
This kind of anchoring is just as impossible when braided or curled yarns are used, because these have an elastic component and therefore with respect to their structure exhibit a distinctly weaker cohesion of the threads. Fibres needed to produce a felt cannot become securely attached to these curled or braided yarns and/or to monofils that have been worked into such yarns, so that under load a migration of the fibres out of the structure formed by curled or braided yarns is practically unavoidable.
In contrast, felts manufactured with a twisted structure in their textile backing fabric exhibit a distinctly improved long-term stability because here, as a result of the firm intertwining or twisting together of the monofils, once the fibres have penetrated into the twisted structure they are permanently anchored there; outward migration is hardly possible and practically never occurs.
According to another embodiment of the invention the twisted structure has a multiply twisted form; that is, in a first step monofils are joined together to form a twisted structure but then several such twisted structures are in turn entwined with one another.
By this means it advantageously becomes possible to affect the above-mentioned stability properties in a specific manner, inasmuch as the fibres necessary to form the felt are given more or fewer possible routes for penetrating between monofils. The anchoring of the felt-like structure in the textile backing element by way of its felt fibres is better, the greater the number of anchoring possibilities available.
Furthermore, a good penetration of the textile backing element by fibres of the felt-like structure has the extremely advantageous effect of providing good transfer of liquid from the side of the felt that faces towards the wet paper through the textile backing element to the side of the felt facing away from the wet paper. Because the transfer of liquid within the felt is based substantially on the capillary forces operating therein, a good penetration of fibres through the textile backing element is crucial for this liquid transfer. Because, as mentioned above, a migration of fibres into or out of the twisted structure practically does not occur, the liquid-transfer performance of the felt is also practically constant over time.
The monofils should have a diameter in the range from 0.1 mm to 0.9 mm, preferably in the range from 0.1 mm to 0.5 mm, and especially preferably in the range from 0.1 mm to 0.3 mm. The diameter in any specific case will depend in particular on the number of monofils incorporated into the twisted structure, the use of three monofils being optimal. In this embodiment the individual monofils have a diameter in the range from 0.2 mm to 0.3 mm.
The twisted structure as a whole has a mean outside diameter in the range from 0.3 mm to 1.0 mm, preferably in the range from 0.4 mm to 0.8 mm and especially preferably in the range from 0.4 mm to 0.6 mm. A mean outside diameter in the range from 0.3 mm to 1.0 mm has proved to be particularly preferable because a twisted structure with this diameter can be optimally integrated into the structure of the textile backing element and hence into the felt.
In this way the disadvantageous tendency of known seamed felts to leave marks can be largely eliminated, so that in operation a felt in accordance with the invention no longer exhibits this tendency.
The felt in accordance with the invention has a transverse-thread density greater than 130 transverse threads per 10 cm, preferably in the range from 130 to 200 transverse threads per 10 cm, and especially preferably in the range from 140 to 180 transverse threads per 10 cm. The result is the extremely advantageous effect that its high transverse-thread density gives the textile backing element an approximately smooth surface, in which unevenness can occur only in the size range of fractions of the diameter of the particular monofil being used. Gaps between the individual transverse threads that would produce inhomogeneity of the textile backing element (for instance, in the form of a wave) are not present in the felt in accordance with the invention. Hence the high transverse-thread density of the textile backing element also creates optimal prerequisites for the felt in accordance with the invention to have no tendency to leave marks on the paper.
Furthermore, the homogeneous construction of the textile backing element and hence of the felt itself largely eliminates the possibility that oscillatory behaviour will be induced, so that even in vibration-sensitive positions of a paper machine the damping potential of the felt is improved in comparison to the state of the art and is preserved in the long term.
On the whole a particular advantage of the invention lies in the fact that the elasticity and/or stability of the felt in accordance with the invention can be optimally adjusted for the particular area of application, for instance the kind of paper to be dried, by suitable choice of the twisted structure.